1. Field of the Invention
The present invention relates to an ultrasonic testing device having a rolling means. Although it may be used in numerous applications, it is more particularly described hereafter with respect to a particular application concerning the non destructive inspection of the bonded junctions of panels forming the skin of the fuselage of an aircraft.
It is known that the skin of the fuselage of an aircraft is formed from individual light alloy panels assembled together by bonded joints, themselves reinforced by rivetted junctions.
Such rivetted and bonded joints are subjected to great strain during the use of the aircraft, more particularly because of the compression and decompression cycles to which the fuselage is subjected. The result may be that zones of loosening develop from small bonding defects not detected during construction. Consequently, the junctions are weakened and it is therefore indispensible to periodically check the bonded junctions so as to know the state thereof.
The complete non destructive inspection of the bonded junctions of the skin of an aircraft could be carried out by means of an ultrasonic probe, emitting or receiving or combining the two functions. In a way known per se, there would be displayed on the screen, for assessing it, an abnormal ultrasonic echo variation caused by a bonding defect, detected in the zone explored, with respect to a normal known and standardized echo of the zone.
However, as is known, the quality of the measurable indications delivered by the usual ultrasonic probes depends on the ultrasonic coupling between the probe and the work piece to be studied. Now, such coupling is provided by a greasy substance (vaseline for example), which must be removed after the measurement by careful cleaning.
Consequently, what is p for a work piece of reasonable size, proves to be unusable when it is a question of aircraft fuselages, particularly large transport aircraft.
The result is then that it is practically impossible to test the junction of an aircraft fuselage using ultrasonic probes, requiring a coupling means between themselves and said fuselage.
2. Description of the Prior Art
Wheeled ultrasonic probes are already known for example from documents U.S. Pat. Nos. 3,628,375 and 4,302,976, intended for rolling over work pieces to be tested and requiring no greasy coupling therewith. Such probes comprise a fixed hub having at least one ultrasonic transducer, as well as an annular rolling means (wheel) mounted for rotation on said hub and surrounding said transducer.
In these known probes, the transducer is housed in a sealed chamber formed between the hub and said wheel and filled with an ultrasonic coupling liquid, internal to said probes. Thus, transmission of the ultrasounds between the transducer and the wheel is provided. However, there are numerous drawbacks:
because of the relative movement of the wheel with respect to the hub, the coupling liquid is "beaten", so that bubbles appear inside said liquid, these bubbles disturbing the measurements;
although seals are provided between the hub and the wheel, leaks of the coupling liquid occur, particularly through the ball or plain bearings by which the wheel is mounted on the hub;
such leaks themselves also cause the formation of bubbles and must then be compensated for; for that, a complementary liquid reservoir is provided, which increases the bulk of said probes.
It follows from the foregoing that such wheeled ultrasonic probes could not be used either for testing the junctions of the fuselage of an aircraft.
A wheeled ultrasonic testing probe is further known, more particularly from the document U.S. Pat. No. 4,472,974, in which the ultrasonic transducer is mounted on the hub through a support whose external face, opposite said transducer, defines with the inner face of said wheel, a rotational sliding slit; in this case, the volume of the coupling liquid contained between the hub and the wheel is smaller than in the probes of documents U.S. Pat. Nos. 3,628,375 and 4,302,976. However, such a construction does not overcome the drawbacks due to the formation of bubbles in the coupling liquid inside the probe and to leaks of this liquid outside the probe.
The purpose of the present invention is to overcome these drawbacks and to provide probes capable of being used more particularly for testing junctions of aircraft fuselages.